The strength of interaction between particles determines whether the substance is a solid, liquid or gas at room temperature.

1. (a)(i)Lithium sulfide, Li₂S, is a crystalline solid with a melting point of 938°C. It conducts electricity when it is molten. Give the formulae of the particles present in solid lithium sulfide.[1]
2. (a)(ii)Explain, in terms of the structure of the crystalline solid, why lithium sulfide has a high melting point.[2]
3. (b)(i)Carbon monoxide, CO, is a gas at room temperature and pressure. It contains a coordinate bond. Explain what is meant by coordinate bond.[1]
4. (b)(ii)Draw a 'dot-and-cross' diagram to show the arrangement of outer electrons in CO. Show the electrons belonging to the C atom as x. Show the electrons belonging to the O atom as •.[2]
5. (c)(i)Nitrogen, N2, is also a gas at room temperature and pressure. Neither CO nor N₂ is an ideal gas. State two assumptions that are made about the behaviour of particles in an ideal gas. 1 2[2]
6. (c)(ii)Explain why N₂ does not behave as an ideal gas at very high pressures.[2]
7. (c)(iii)Complete the table by naming all the types of intermolecular forces (van der Waals') in separate samples of N₂(g) and CO(g). [Table with N2(g) and CO(g) data for electrons, dipole moment, boiling point, and blank row for intermolecular forces] number of electrons per molecule N2(g) 14 CO(g) 14 presence of a dipole moment X ✓ boiling point/°C -195.8 -191.5 intermolecular forces (van der Waals')[2]
8. (c)(iv)Suggest why the bond in a molecule of CO contains a dipole moment.[1]

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